If you don't have a console or monitoring device that has a simple Mono switch, then you would have to sum in PT. The only way to do this is to bus everything to an aux and change the stereo panning of both L/R to '0' (or you could use sends on all tracks to a seperate mono aux which you can mute or solo). But you can also check out Brainworx RTAS plug-ins - they make some really great M-S plug-ins, and they have one called Solo (for FREE!) that you can put on the master fader and up the "Mono-Maker" knob to listen in mono. You can also solo just the mid or side (sum and difference) of your session. All-in-all, handy stuff. Check them out!
MS-Mastering Tools by Brainworx-Media
Solo can be downloaded here:
MS-Mastering Tools by Brainworx-Media
I've dealt with Dirk personally, and he's a really awesome guy - can't wait to see some of their new products.